Digital halftoning is a technique for displaying a picture on a two-dimensional medium, in which small dots and a limited number of colors are used. The picture appears to consist of many colors when viewed from a proper distance. For example, a picture consisting of black and white dots can appear to display various gray levels. Digital printers, which were initially pure black and white machines with a very coarse resolution, have evolved to accommodate colors, finer resolutions, and more recently, more than one bit of information per pixel (referred to as “multi-bit” or “multi-tone”).
Screening is a type of halftoning method used commonly in practical implementations. A binary screening method employs a 3D Look-Up table (LUT) replicated to the size of printable area containing the binary output values for every PEL of the array and for every gray level of the device. These replicated LUTs are indexed by the Continuous Tone Image (CTI) data to determine which PELs are ON or OFF. A print controller receives a CTI, such as a digital picture, from a host. The print controller then uses the screening algorithm to process the CTI and convert the image into an array of pixels. The result of the screening algorithm is a bitmap where each pixel may be ON or OFF, which is referred to as a Half-Tone Image (HTI). The print controller then sends the HTI to a print engine for printing.
Conventional digital halftoning techniques are designed as a function of either the dot size [amplitude modulation (AM)] or the dot density [frequency modulation (FM)]. Generally, AM halftoning methods have the advantage of low computation and good print stability for electro photographic printers, while FM halftoning methods typically have higher spatial resolution and resistance to moiré artifacts and are used in inkjet printers.
New classes of AM/FM (e.g., hybrid) halftoning algorithms exist that simultaneously modulate the dot size and density. The major advantages of hybrid halftoning are stable as AM halftones, moiré resistance as FM methods through irregular dot placement, and improved quality through systematic optimization of the dot size and dot density at each gray level.
With the prevalence of devices having multi-bit capability there is a potential to improve overall image quality of print jobs using multi-bit halftoning. Multi-bit screening enables a selection among multiple drop sizes or exposure levels at each addressable pixel. The 3D LUT approach can be extended to represent multibit drop sizes or exposure values by using these values in the LUT instead of binary values.
The planes of the 3D Look-Up Table (LUT) form a 3D array having planes representing the halftone patterns for each darker gray level, ranging from the pattern for gray level zero through the maximum gray level of the halftone mask. The maximum gray level is used to produce a solid, where all of the pixels are printed at the maximum output state.
Accordingly, an algorithm to efficiently generate multi-bit hybrid halftone screens is desired.